1. Field of the Invention
The present invention relates to a differential varactor device.
2. Description of the Prior Art
In the modern information industry, all kinds of data, information, video, and so on are all transmitted electronically; therefore, a processing circuit for dealing with electronic signals becomes one of the most important foundations of modern information business. An oscillator is an indispensable circuit block for modern digital circuits. For example, in common information systems (such as a personal computer), a global clock is required to coordinate all digital circuits in the system, so an oscillator for generating clock is required. In addition, to synchronize circuits with different clocks, phase loop lock (PLL) circuits are needed, and a precise voltage-controlled oscillator (VCO) is essential for the PLL to generate different frequencies of signals. In VCOs, the frequency of the VCO is controlled by an applied current or voltage. Furthermore, in some precise filters, resistor-capacitor (RC) filters, in which the filter frequency can be adjusted, are utilized frequently.
With the filter characteristic of an RC filter and the oscillation characteristic of an inductance-capacitor (LC) oscillator, it is possible to adjust each of them by modifying the capacitance value. In devices with those characteristics, capacitors with variable capacitances, which are varactor structures, are used. The capacitance of a varactor structure, when within its operating parameters, decreases as a voltage applied to the device (the control voltage) increases. Numerous varactor structures have been developed and are employed in integrated circuit technologies. Among them, PN junction varactor structures and metal-oxide-semiconductor (MOS) varactor structures are commonly used.
However, regarding the MOS varactor, the tuning ratio, which is defined as a ratio of the largest capacitance and the smallest capacitance of the MOS varactor, only is 3 in the operating frequency of 10 GHz. In other words, the range of the adjustable capacitance of the MOS varactor is limited by the tuning ratio. Therefore, to increase the tuning ratio of the varactor is an important objective in this field.